Being one of the best characterized coat complexes, Coat Protein I (COPI) has been a model to elucidate fundamental mechanisms of how coat proteins drive the initial step of intracellular transport - the formation of transport carriers. W have been at the forefront in studying COPI transport, and propose four lines of investigation in further defining its mechanisms and physiology. First, in studying how key lipids act in fission, which is the process that completes vesicle formation, we have recently uncovered a new principle for how lipid geometry promotes membrane curvature to drive the late stage of COPI vesicle fission. Thus, we will determine whether this novel mechanism also operates in early fission. We will also determine whether the key factors that we have identified thus far to act in COPI vesicle fission will allow us to reconstitute this process using purified components. Second, we have recently discovered that COPI generates not only vesicles but also tubules. Whereas COPI vesicles act in retrograde Golgi transport, COPI tubules act in anterograde Golgi transport. Mechanistically, we have further elucidated that cdc42 promotes COPI tubule formation through an intrinsic ability to generate membrane curvature. Thus, we will seek a molecular understanding of this novel role. We will also explore how other Rho family members act in bidirectional COPI transport by targeting the two major functions of COPI - carrier formation and the sorting of cargoes into transport carriers. Third, we note that the KDEL receptor has also been found to regulate bidirectional Golgi transport, which has been elucidated recently to involve signaling through G proteins. However, as the KDEL receptor does not possess structural features typical of G protein-coupled receptors, we will elucidate how it couples ligand binding with recruitment of G proteins. Fourth, we have recently found that glyceraldehyde-3- phosphate dehydrogenase (GAPDH) inhibits COPI transport. As evidence suggests that it targets COPI vesicle fission, we will determine how this is achieved. Additional evidence suggests that the inhibition of COPI transport by GAPDH acts in energy balance. Thus, we will also further elucidate how GAPDH achieves this role. We anticipate that the completion of these four aims will advance a fundamental understanding of how COPI acts to generate transport carriers, as well shed new insights into the physiology of this transport. Moreover, as intracellular transport is a basic cellular process that underlies physiology and pathologic states, our studies will contribute to a better understanding of disease mechanisms.